There's no such thing as CSS5; the latest version of CSS is 3. Also, there's not much of a difference from HTML5 and it's previous versions, aside from a few different things. Obviously you should strive to implement the standards of the latest version.
Yeah, not sure where you're getting CSS5 from, lol.
Anyway, you'll find that most web elements are HTML 4.0 anyway, it's just there are certain elements added that are classed as HTML5 (
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). Although, saying that, it's probably best to avoid some old HTML 4.0 methods as they're deprecated (
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) - which leads on to to CSS3..
The above tags are deprecated mainly for mobile purposes and to encourage more styling to be via a stylesheet (which makes more sense) and this is where the extra support of CSS3 comes in, which is widely support by virtually ever modern browser (I think ie8+ supports it but don't quote me on that) - you can see a list of the css3 selectors
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Hope this is some use, you'll find you are probably already using many of these tags/selectors as you learn - most front-end tutorials use it by default.